1. Field of the Invention
The present invention relates generally to the field of electronic filters. In particular, the present invention relates to an echo canceller with the ability to cancel the echo response caused by non-linearities. While the present invention has numerous other applications, it will be described in relation to its application in an integrated services digital network (ISDN) system.
2. Background Information
The telephone system has expanded to such a degree that today it is possible for almost anybody to have almost instantaneous communications with almost anybody else almost anywhere in the world. Since the phone system is so ubiquitous, it provides a desirable communication medium for more than just voice-to-voice communication. Additional services such as facsimile transfers, telemail, electronic funds transfer, video data, computers links, and alarm systems benefit from the use of the telephone system.
Using the existing telephone system as a communication channel for the additional services involves more than just connecting a machine to a phone line. For example, the existing phone system was only designed to transmit analog signals with a bandwidth of about 300-3500 hertz, sufficient for voice communication but not optimum for the additional services which generally transmit data having significantly higher frequency components. The result is that the signals of the additional services distort as they propagate.
Aggravating the problem of using the present system is the fact that only a single twisted pair of wires interconnect a subscriber (user) with a central office. This means that the subscriber's transmitter (mouthpiece) and receiver (earpiece) share the same two wires. While this is a low cost method of connecting subscribers to a central office, because full duplex (simultaneous two-way) operation is desired, cancellation of a subscriber's transmission from his own receiver is required. This is understood by considering that if a first and second subscriber both transmit at the same time, the second subscriber's signal attenuates as it travels toward the first subscriber. If the first subscriber does not cancel out of his receiver his own transmission he will hear only his own transmission and not the desired, attenuated second subscriber's signal.
Full duplex operation on two wires therefore requires that a transmitted signal must be reduced sufficiently from the transmitter's own receiver to allow reception of an incoming signal. While readily accomplishable with voice communication, it is much harder to do with the additional services because those services are more sensitive to incompletely cancelled signals.
Exacerbating the problem of increased sensitivity to incompletely cancelled signals are the numerous wire taps, wire gauge changes, and switching networks which cause signal "reflections" in the present phone system. These reflections can be picked up by the transmitter's receiver and, if not handled properly, could be mistaken for a signal from another subscriber.
Even further complicating the use of the present phone system as a communication medium is that the present phone lines have widely variable transmission line characteristics. This creates a problem because any mismatch between the telephone line and the service using the phone line causes an "incident" signal that is reflected into the receiver.
Even the manner of transmitting digital data on a phone line can create problems. For example, when the digital data is converted to a form suitable for transmission on the phone line, errors in conversion may be amplitude sensitive in a nonlinear manner. Other sources of nonlinear error are described below.
Adding together the reflections, incident signals, and nonlinearities, one obtains the "transhybrid response." The transhybrid response is then the complete echo response verses time caused by a transmitted signal. The transhybrid response has components which are linear functions of the transmitted signal, these will be referred to as linear echoes and components which are nonlinear functions of the transmitted signal, which will be referred to as nonlinear echoes.
The effects of insufficient transmission cancellation, line taps, mismatches, high frequency, and nonlinearities are very serious. To assist in dealing with these and other problems, the Accredited Standards Committee on Telecommunications, T1, submitted to the American National Standards Institute a standard for integrated networking, ANSI T1.601-1988, entitled "Integrated Services Digital Network (ISDN)--Basic Access Interface for Use on Metallic Loops for Application on the Network Side of the NT (Layer 1 Specification)." This document describes a set of requirements and protocols for digital communication when using full duplex operation on the existing single twisted wire pair phone line.
A signal transmitted on a phone line may become attenuated by as much as 40 db (100 times), making a 5/6 volt transmit signal only about 0.0085 volts at the receiver. Since an echo may be almost as large as a transmit signal, or up to about 2.5 volts, a receiver may be required to detect a 0.0085 volt signal riding on a 2.5 volt echo. Reduction of the echo amplitude to an acceptable amount is the job of the echo canceller.
While the echo canceller must reduce the echo to an acceptable amount (roughly a 60 db reduction), the actual transhybrid response is unknown until the lines connecting the subscribers is established, something that does not occur until a call is answered. Therefore, acceptable methods of echo cancelling must be adaptive, i.e. they must adjust to the line conditions existing at the time of the call.
One commonly used method of echo cancelling was proposed by Kurt H. Mueller in "A New Digital Echo Canceller for Two-Wire Full-Duplex Data Transmission" found in the IEEE Transactions on Communications, Sep. 1976, at pp. 956-962. Mr. Mueller proposed using a transversal filter to eliminate the transhybrid response by adjusting the filter's output to cancel the response. An improved transversal filter based echo canceller was described in co-pending U.S. Pat. application No. 07/593, filed Apr. 10, 1990 assigned to the assignee of the present application and is hereby incorporated by reference.
A problem with transversal filter based echo cancelers is that they are ineffective against echo components caused by nonlinearities. This was noted by Mr. Ernst G. Peter in "Multistage RAM: An FIR Filter for Echo Cancellation in a Digital Two-Wire Subscriber Loop" given in the IEEE Transactions on Circuits and Systems, vol 34 no. 3 Mar. 1987, page 225-232. Several sources of these nonlinearities were given by Agazzi, Messerschmitt, and Hodges in "Nonlinear Echo Cancellation of Data Signals" in IEEE Transactions on Communications, vol 30, no. 11, Nov. 1982. These sources included nonlinearities existing in A/D or D/A converters, unsymmetric transmitted signals, and transformer hysteresis and saturation.
Eliminating nonlinear echoes is complicated by the fact that the echo levels tend to be complex functions of the transmitted signal's amplitude. Mr. Ernest G. Peter, in his above mentioned article, indicated that one method of reducing nonlinear echoes was to use the "Look-up table" approach.
In the Look-up table approach all possible combinations of the previous N bauds of transmitted data are stored in a random access memory (RAM) look-up table. Each possible combination is then given its own cancellation magnitude which is used to cancel the signal whenever that pattern occurs. While the look-up table approach can reduce nonlinear echoes, it has the serious drawbacks of a slow operating time and requiring a large amount of memory when N gets large.
Agazzi, Messerschmitt, and Hodges in "Nonlinear Echo Cancellation of Data Signals" in IEEE Transactions on Communications, vol 30, no. 11, Nov. 1982 presented an algorithm which suggested an echo canceller having the characteristics of a look-up table approach but without requiring an excessive amount of memory. The possibility of spatially separate linear and nonlinear echo cancelers is specifically suggested in the text. However, Agazzi, Messerschmitt, and Hodges do not address how the separate echo cancelers could operate together when it is possible that one echo canceller could adjust its output in the opposite manner of the other canceller, thus causing problems of convergence.
It is therefore useful and desirable to reliably reduce both linear and nonlinear echoes in an adaptive fashion without using an excessive amount of memory or without requiring excessive time for the canceller to operate.